Traffic challenge not primarily intersection
To the editor:
Roundabouts work best when traffic volumes are balanced and when there is sufficient capacity beyond the intersection to absorb vehicles as they exit. Sanibel’s traffic pattern is the opposite. The island experiences large, directional surges of traffic, particularly in the late afternoon when workers, visitors and residents are all trying to leave at roughly the same time. Because the Sanibel Causeway limits outbound traffic to one lane, vehicles cannot clear the island any faster than that lane allows. Installing roundabouts in this environment does not increase throughput; it only rearranges where the congestion occurs.
In fact, roundabouts can introduce additional complications in a high-volume, single-exit system like Sanibel’s. When traffic is heavily backed up leaving the island, vehicles entering the roundabout from other directions may find no safe gap to merge. This can lead to gridlock conditions inside the circle itself, where drivers hesitate or block entrances, further reducing efficiency. Large service vehicles, delivery trucks and emergency vehicles — all common on Sanibel — can also have difficulty navigating tight roundabout designs when traffic is dense.
Sanibel’s geography and transportation system are unique. The island functions less like a typical grid of interconnected streets and more like a cul-de-sac with one door in and one door out. Because the causeway ultimately determines the maximum rate at which vehicles can leave, improvements to internal intersections cannot meaningfully increase overall traffic capacity. In engineering terms, the system is constrained by a downstream choke point. Until that constraint changes, any intersection design — roundabout or otherwise — will still be governed by the single-lane limit of the causeway.
For this reason, roundabouts are unlikely to provide the benefits sometimes promised in traffic studies that assume continuous flow beyond the intersection. On Sanibel, the reality is that traffic leaving the island will always compress into one lane on the causeway. When that lane reaches capacity, congestion will occur regardless of how efficiently vehicles move through internal intersections. The island’s traffic challenges are therefore not primarily an intersection problem — they are a capacity constraint at the causeway.
Bob Brooks
Sanibel